Wafer processing with carrier extension

ABSTRACT

Apparatus for treating wafers using a wafer carrier rotated about an axis is provided with a ring which surrounds the wafer carrier during operation. Treatment gasses directed onto a top surface of the carrier flow outwardly away from the axis over the carrier and over the ring, and pass downstream outside of the ring. The outwardly flowing gasses form a boundary over the carrier and ring. The ring helps to maintain a boundary layer of substantially uniform thickness over the carrier, which promotes uniform treatment of the wafers.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of the filing date of U.S.Provisional Application No. 61/428,250, filed Dec. 30, 2010, thedisclosure of which is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to wafer processing apparatus, to wafercarriers for use in such processing apparatus, and to methods of waferprocessing.

Many semiconductor devices are formed by processes performed on asubstrate. The substrate typically is slab of a crystalline material,commonly referred to as a “wafer.” Typically, the wafer is formed from acrystalline material, and is in the form of a disc. One common processis epitaxial growth. For example, devices formed from compoundsemiconductors such as III-V semiconductors typically are formed bygrowing successive layers of the compound semiconductor using metalorganic chemical vapor deposition or “MOCVD.” In this process, thewafers are exposed to a combination of gases, typically including ametal organic compound as a source of a group III metal, and alsoincluding a source of a group V element, which flow over the surface ofthe wafer while the wafer is maintained at an elevated temperature.Typically, the metal organic compound and group V source are combinedwith a carrier gas which does not participate appreciably in thereaction as, for example, nitrogen. One example of a III-V semiconductoris gallium nitride, which can be formed by reaction of an organo galliumcompound and ammonia on a substrate having a suitable crystal latticespacing, as, for example, a sapphire wafer. Typically, the wafer ismaintained at a temperature on the order of 1000-1100° C. duringdeposition of gallium nitride and related compounds.

Composite devices can be fabricated by depositing numerous layers insuccession on the surface of the wafer under slightly different reactionconditions, as, for example, additions of other group III or group Velements to vary the crystal structure and bandgap of the semiconductor.For example, in a gallium nitride based semiconductor, indium, aluminumor both can be used in varying proportion to vary the bandgap of thesemiconductor. Also, p-type or n-type dopants can be added to controlthe conductivity of each layer. After all of the semiconductor layershave been formed and, typically, after appropriate electric contactshave been applied, the wafer is cut into individual devices. Devicessuch as light-emitting diodes (“LEDs”), lasers, and other optoelectronicdevices can be fabricated in this way.

In a typical chemical vapor deposition process, numerous wafers are heldon a device commonly referred to as a wafer carrier so that a topsurface of each wafer is exposed at the top surface of the wafercarrier. The wafer carrier is then placed into a reaction chamber andmaintained at the desired temperature while the gas mixture flows overthe surface of the wafer carrier. It is important to maintain uniformconditions at all points on the top surfaces of the various wafers onthe carrier during the process. Variations in process conditions cancause undesired variations in the properties of the resultingsemiconductor device. For example, variations in the rate of depositioncan cause variations in thickness of the deposited layers, which in turncan lead to non-uniform characteristics in the resulting devices. Thus,considerable effort has been devoted in the art heretofore towardsmaintaining uniform conditions.

One type of CVD apparatus which has been widely accepted in the industryuses a wafer carrier in the form of a large disc with numerouswafer-holding regions, each adapted to hold one wafer. The wafer carrieris supported on a spindle within the reaction chamber so that the topsurface of the wafer carrier having the exposed surfaces of the wafersfaces upwardly toward a gas distribution element. While the spindle isrotated, the gas is directed downwardly onto the top surface of thewafer carrier and flows across the top surface toward the periphery ofthe wafer carrier. The outwardly-flowing gas forms a boundary layercovering the top surface of the wafer carrier. The used gas flowsdownwardly around the periphery of the wafer and is evacuated from thereaction chamber through ports disposed below the wafer carrier. Thewafer carrier is maintained at the desired elevated temperature byheating elements, typically electrical resistive heating elementsdisposed below the bottom surface of the wafer carrier.

The rate of certain treatment processes, such as the growth rate in anMOCVD process under mass-transport-limited growth conditions, isinversely related to the boundary layer thickness. For the case of aninfinitely large carrier, theory predicts that the rate is inverselyproportional to the boundary layer thickness. This means that forthinner boundary layers the growth rate is higher. This reflects thefact that, as the boundary layer becomes thinner, it takes less time forreactive moieties to diffuse through the boundary layer to the surfaceof the wafer carrier and the surfaces of the wafers. Hence, a thin anduniform diffusion boundary layer is desirable to achieve uniform andfast deposition rate during the MOCVD epitaxial growth. Boundary layerthickness can be controlled by changing the rotation rate and pressurein reactor and is inversely proportional of the square root of those twoparameters. It can also be controlled by changing the dynamic viscosityof the gas mixture. The dynamic viscosity is a function of fraction ofdifferent gases in the mixture as well as of carrier and inlettemperature.

Typically, with stable flow conditions in the reactor and withsubstantially uniform heating of the wafer carrier, uniform boundarylayer thickness can be achieved above the majority of the wafer carriersurface. However, near the periphery of the wafer carrier, the gas flowbegins to change direction from radial above the wafer carrier to thedownward flow which carries the gas from the wafer carrier to theexhaust. In the edge region of the wafer carrier near the periphery, theboundary layer becomes thinner and hence the process rate increasesappreciably. For example, if a wafer is positioned on the carrier with aportion of the wafer disposed in the edge region, a chemical vapordeposition process will form layers of uneven thickness. Thickerportions will be formed on those parts of the wafer disposed in the edgeregion.

To avoid this problem, wafers are not positioned in the edge region.Thus, the pockets or other wafer-holding features of wafer carrierstypically are provided only in the area of the wafer carrier remote fromthe periphery. This limits the number and size of wafers which can beaccommodated on a carrier of a given size, and therefore limits theproductivity of the equipment and process. Although a larger wafercarrier could accommodate more wafers, larger carriers have significantdrawbacks. Larger carriers are more expensive, heavier and thus moredifficult to handle, particularly during movement of the carrier intoand out of the reaction chamber. Moreover, it typically is impracticalto increase the size of the wafer carriers used in existing processingequipment.

Thus, although considerable effort has been devoted in the artheretofore to design an optimization of such systems, still furtherimprovement would be desirable.

SUMMARY OF THE INVENTION

One aspect of the invention provides a reactor. The reactor according tothis aspect of the invention desirably includes a chamber having a wallstructure defining an interior surface. The reactor preferably has aspindle disposed within the chamber and rotatable about anupstream-to-downstream axis, the spindle being adapted to support awafer carrier for rotation about the axis so that a top surface of thecarrier faces in the upstream direction at a carrier location. Thereactor according to this aspect of the invention preferably alsoincludes a ring mounted within the chamber, the ring having a topsurface facing in the upstream direction, the ring being constructed andarranged so that when the reactor is in an operative condition, the ringclosely surrounds the wafer carrier supported on the spindle and the topsurface of the ring is substantially continuous with the top surface ofthe carrier. The ring typically is movably mounted within the chamber,so that it does not impede loading or unloading of carriers.

Typically, the reactor also includes a gas inlet element communicatingwith the chamber upstream of the carrier location and a gas exhaustcommunicating with the chamber downstream of the carrier location. Thering typically has a peripheral surface facing outwardly away from theaxis, the ring being arranged so that when the reactor is in anoperative condition, there is a gap between the peripheral surface ofthe ring and the interior surface of the chamber. As further discussedbelow, during operation of the reactor, gas discharged from the gasinlet element flows downstream toward the wafer carrier and over the topsurface of the carrier and wafers held on the carrier, and flowsoutwardly over the ring. In effect, the ring forms an extension of thecarrier, so the gas flow is similar to that which would be obtained witha carrier of larger diameter. The boundary layer may have substantiallyuniform thickness over the entire carrier, or over almost the entirecarrier, so that wafer parts or wafers can be positioned in edge regionsof the carrier.

A further aspect of the invention provides methods of processing wafers.The method according to this aspect of the invention desirably includesthe step positioning a wafer carrier inside a reaction chamber so that aring within the chamber surrounds the carrier, so that top surfaces ofthe carrier and ring facing in an upstream direction are substantiallycontinuous with one another and so that surfaces of wafers disposed onthe carrier face in the upstream direction. The method preferably alsoincludes the step of directing one or more treatment gasses in adownstream direction onto the top surfaces of the wafer carrier andwafers while rotating the wafer carrier and wafers around anupstream-to-downstream axis, so that the treatment gasses flow outwardlyover the top surface of the carrier and the surfaces of the wafer, andflow outwardly from the top surface of the carrier over the top surfaceof the ring. The method typically further includes exhausting gassesfrom the chamber downstream from the ring so that the gasses flowingoutwardly over the top surface of the ring pass downstream within a gapbetween the ring and a wall of the chamber.

Yet another aspect of the invention provides wafer carriers. Wafercarriers according to this aspect of the invention desirably include abody having a circular top surface, a peripheral surface bounding thetop surface and a fitting adapted to engage a spindle of a waferprocessing reactor so that the top surface and peripheral surface areconcentric with the spindle. The body desirably further defines aplurality of pockets each adapted to hold a wafer, the pockets includingouter pockets adapted to hold wafers so that portions of the wafers liewithin about 5 mm of the peripheral surface.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the subject matter of the presentinvention and of the various advantages thereof can be realized byreference to the following detailed description in which reference ismade to the accompanying drawings in which:

FIG. 1 is a diagrammatic sectional view of apparatus according to oneembodiment of the invention.

FIG. 2 is a fragmentary view of the region indicated at 2 in FIG. 1.

FIG. 3 is a fragmentary view taken along line 3-3 in FIG. 2.

FIG. 4 is a view similar to FIG. 2, but depicting a portion ofconventional apparatus according to the prior art.

FIG. 5 is a graph of predicted performance for apparatus according toFIGS. 1-3 and for the apparatus of FIG. 4.

FIG. 6 is a view similar to FIG. 2, but depicting a portion of apparatusaccording to a further embodiment of the invention.

FIG. 7 is a diagrammatic top view of a wafer carrier according to afurther embodiment of the invention.

FIG. 8 is a view similar to FIG. 2, but depicting a portion of apparatusaccording to yet another embodiment of the invention.

FIG. 9 is a view similar to FIG. 6, but depicting a portion of apparatusaccording to yet another embodiment of the invention.

FIG. 10 is a diagrammatic view of apparatus according to yet anotherembodiment of the invention.

FIG. 11 is a top plan view of the assembly of FIG. 10.

DETAILED DESCRIPTION

In describing the preferred embodiments of the subject illustrated andto be described with respect to the drawings, specific terminology willbe used for the sake of clarity. However, the invention is not intendedto be limited to any specific terms used herein, and it is to beunderstood that each specific term includes all technical equivalents,which operate in a similar manner to accomplish a similar purpose.

Apparatus according to one embodiment of the invention incorporates areaction chamber 10 having a wall structure which incorporates a fixedwall 12 defining a generally cylindrical space 15 having a central axis14 and an opening 16 communicating with the interior space. As furtherdiscussed below, gas flow within the reaction chamber during operationis generally from the region at the top of the drawing in FIG. 1 towardthe region at the bottom of the drawing. Therefore, the direction alongthe axis toward the bottom of the drawing, indicated by arrow D in FIG.1, is referred to herein as the “downstream” direction, and the oppositedirection denoted by arrow U is referred to herein as the “upstream”direction.

The wall structure of the chamber further includes a ring-like shutter18. The shutter 18 has a central axis coincident with the central axis14. Shutter 18 is mounted for movement relative to the fixed wall in theupstream and downstream directions and is connected to a movementactuator 20. Actuator 20 is arranged to move the shutter between theoperative position illustrated in solid lines in FIG. 1 and the openposition depicted in broken lines at 18′ in FIG. 1. When shutter 18 isin the operative position, it covers opening 16. Typically, shutter 18does not form a gas-tight seal at opening 16. Fixed wall 12 and shutter18 are provided with coolant passages (not shown) inside the walls or ontheir exterior surfaces. The coolant passages are connected to a coolantsupply apparatus (not shown), so that the fixed wall and shutter can bemaintained at desired temperatures during the process.

A gas inlet element 22 is provided at an upstream end of chamber 10,towards the top of the drawing in FIG. 1. The gas inlet element isconnected to one or more sources 24 arranged to supply one or moretreatment gases. Gas inlet element 22 may be generally conventional andmay be arranged to discharge the treatment gases in a flow directedgenerally in the downstream direction D. The gas inlet element typicallyis arranged to discharge the treatment gases in a pattern of dischargesspaced around central axis 14 and distributed at various radialdistances from the central axis. The gas inlet element typically is alsoprovided with coolant channels (not shown) for maintaining itstemperature during the process.

A hollow hoop-like exhaust manifold 26 is provided adjacent thedownstream end of the chamber. The exhaust manifold has an interiorpassage 28 and numerous ports 30 open to the interior of the chamber.The interior passage 28 of the exhaust manifold, in turn, is connectedto an exhaust system 32 arranged to pump gases out of the interior space15 and discharge the gases to waste.

A spindle 34 is mounted to the fixed wall structure 12 for rotationabout central axis 14. Spindle 34 is connected to a rotary drivemechanism 36. The spindle has a fitting 38 at its upstream end. Thefitting is arranged to releasably engage and hold a wafer carrier 40 atthe carrier location depicted in FIG. 1. The carrier location isdisposed downstream from gas inlet element 22, but upstream from exhaustmanifold 26. A heater 42 is disposed downstream from the carrierlocation and surrounds spindle 34. Heater 42 is supported within thechamber by supports (not shown) fixed to the fixed wall structure 12. Acircular baffle 44 surrounds the heater and extends downstream from thecarrier location. A source 45 of a heater purge gas communicates withthe space inside of baffle 44. As best seen in FIG. 2, the baffle isdimensioned so that, when a wafer carrier 40 is mounted at the carrierlocation, there is a small gap 47 between the baffle and the carrier.During operation, the heater purge gas source 45 feeds a purge gas, suchas nitrogen, into the space within baffle 44 so that the purge gas flowsout of this space through gap 47 and passes to exhaust 32 along with theother gas flows discussed below. The heater purge gas prevents thetreatment gas from contacting and attacking heater 42.

An antechamber 48 communicates with opening 16 in the fixed wallstructure. Antechamber 48 is provided with a closure, such as a gatevalve element 50, schematically shown in FIG. 1. The gate valve elementis arranged to seal the antechamber and thus block communication betweenantechamber 48 and the interior space 15. Valve element 50 can be movedto a retracted position (not shown) to allow communication between theantechamber and interior space 15. When the valve element is in theretracted position and shutter 18 is in the open position 18′, a wafercarrier 40 can be removed from its engagement with fitting 38 of thespindle and moved through opening 16 into the antechamber using robotichandling apparatus (not shown). A new wafer carrier 40′ can be movedfrom the antechamber into the reaction chamber and engaged with fitting38 so that the new wafer carrier is positioned at the carrier location.

A ring 52 is mounted to shutter 18 and thus positioned within theinterior space 15 of the chamber. As best seen in FIGS. 2 and 3, ring 52has a top surface 54 facing in the upstream direction; an outercircumferential surface 56 facing radially outwardly, away from thecentral axis; and an inner surface 58 facing radially inwardly, towardthe central axis. Ring 52 is mounted to shutter 18 by struts 60 disposedaround the circumference of the chamber. One such strut is depicted inFIGS. 2 and 3. The struts are disposed below top surface 54. The outerperipheral surface 56 of the ring is disposed radially in-board of theadjacent surface of shutter 18, so that there is a gap 62 between thesurface of the shutter and the ring. For example, in apparatus arrangedto hold a 465 mm diameter wafer carrier, the width of gap 62 at itsnarrowest point may be on the order of 13 mm. Because struts 60 arerelatively thin, they do not materially obstruct gap 62. The dimensionsof ring 52 and its mounting to shutter 18 are selected so that when theshutter 18 is in an operative condition, as shown in solid lines in FIG.1 and as depicted in FIG. 2, and when a wafer carrier 40 is disposed inan operative condition and positioned at the carrier location inengagement with fitting 38 on spindle 34, the top surface 54 of the ringis substantially coplanar with the top or upstream surface 64 of thecarrier 40. The width or radial extent of ring 52 desirably may be about13-15 mm, and still greater ring widths are more desirable. In general,ring 52 should be as wide as is practicable. Where the ring is to befitted into existing systems originally built without the ring, the ringwidth is limited by the need to provide a gap 62 of sufficient width.

Also, ring 52 is dimensioned and mounted so that in this operativecondition, the interior surface 58 of the ring lies adjacent theexterior peripheral surface 66 of the wafer carrier 40, leaving only asmall gap 70 between the surfaces. Desirably, gap 70 is as small aspracticable, consistent with manufacturing tolerances and allowances fordifferential thermal expansion of the components. For example, gap 70may be about 2 mm wide or less. Preferably, the cross-sectional area ofgap 70 is less than about 5% of the cross-sectional area of gap 62between the exterior peripheral surface of the ring and the shutter 18,as measured at the narrowest point of gap 62.

As best seen in FIGS. 1 and 2, each wafer carrier 40 defines numerouspockets 72, each of which is arranged to hold a wafer 74 so that a topsurface of the wafer is substantially coplanar with the top surface 64of the carrier. Desirably, wafer carrier 40 has a relatively sharp edgeat the juncture of its top surface 64 and peripheral surface 66, andring 52 desirably also has sharp edges at the juncture of its topsurface with interior surface 58 and exterior peripheral surface 56.These sharp edges desirably have radii less than about 0.1 mm.

In operation, the apparatus is brought to the operative conditiondepicted in FIGS. 1-3, with a wafer carrier 40 bearing wafers 74disposed on the spindle and with the shutter 18 in the operativeposition shown in solid lines, so that ring 52 closely surrounds theperipheral surface of carrier 40. Heater 42 is actuated to bring thewafer carrier and wafers to the desired temperature, and gas inletelement 22 is actuated to discharge treatment gases, while rotary drive36 is actuated to spin the spindle 34 and wafer carrier 40 about centralaxis 14. The gas discharged by gas inlet element 22 passes generally asindicated by flow arrows F in FIG. 1. Thus, the gas passes downstreamfrom the inlet element towards the carrier location and flows generallyradially outwardly over the top or upstream surface of carrier 40. Theflowing gas passes outwardly beyond the periphery of the wafer carrierand over ring 52, and then passes downwardly through the gap 62 betweenthe ring and the interior wall surface defined by shutter 18. Although aminor amount of the gas passes downwardly through gap 70, this minoramount does not substantially influence the flow dynamics of the system.Preferably, less than about 5% of the gas passing over the top surfaceof the wafer carrier passes through gap 70, and the remainder passesthrough gap 62, out-board of the ring 52. The gas continues to flowdownstream towards exhaust manifold 26 and passes into the exhaustmanifold and out from the system through exhaust system 32.

As best seen in FIG. 2, the gas flowing outwardly over top surface 64 ofthe wafer carrier and over the surfaces of the wafers 74 forms aboundary layer B. Within this boundary layer, the gas flow streamlinesare nearly parallel to the top surface of the carrier, so that theboundary layer has a substantially uniform thickness. However, as thegas approaches the gap 62, the streamlines converge appreciably in aregion R, and the thickness of the boundary layer decreases appreciablywithin this region. However, this region is disposed over the ring 52and not over the wafer carrier. Therefore, the boundary layer maintainsa substantially uniform thickness over substantially the entire topsurface of the wafer carrier. This provides a substantially evenreaction rate over surfaces of wafers 74, even when the wafers 74 aredisposed immediately adjacent the peripheral surface 66 of the carrier.

After processing, shutter 18 is moved to the open 18′ configuration.Ring 52 moves with the shutter to the position shown at 52′ in FIG. 1.When the shutter is in the retracted position, both the ring and theshutter are remote from opening 16 and do not impede movement of wafercarriers into and out of the chamber.

FIG. 4 depicts a system identical to the system shown in FIG. 2, butwithout ring 52, and using a typical wafer carrier having an appreciableradius at the juncture between the top surface of the carrier and theperipheral surface of the carrier. In this system, the gas passesdownstream immediately outside the peripheral surface 66 of the wafercarrier. Thus, the streamlines converge appreciably over the outerportion of the wafer carrier itself. The region R of uneven boundarylayer thickness extends inwardly from the peripheral surface 66 of thewafer carrier and covers a significant portion of the carrier topsurface. Thus, if parts of wafers 74 are positioned within the area ofthe carrier covered by region R, these wafers will be subjected touneven growth rates. Thus, in a system without ring 52, thewafer-holding pockets typically would be positioned differently, so asto keep them further away from the periphery of the carrier. This, inturn, would reduce the capacity of the wafer carrier. Stated anotherway, the presence of ring 52 (FIGS. 1-3) allows placement of the wafercarrier pockets close to the periphery of the carrier and thus increasesthe capacity of the carriers. This increases the throughput of thesystem, i.e., the number of wafers which can be processed per unit time.

Moreover, placing wafers closer to the periphery of the carrier promotesefficient use of the treatment gases. These gases typically areexpensive, high-purity materials. Typically, the amount of each gas isdetermined so as to provide a constant amount per unit area over theentire area of the wafer carrier. By placing wafers closer to theperiphery of the carrier, more of the area of the carrier can be coveredby wafers, and more of the gas will be used to treat wafers.

The effect of the change in flow dynamics introduced by addition of ring52 is further shown in FIG. 5. Curve 100 in FIG. 5 represents acalculated plot of thickness versus radial position in a chemical vapordeposition process using the reactor depicted in FIG. 4, with no ringand with a wafer carrier having a radiused edge. Curve 102 is a similarplot of calculated deposition thickness in the same chemical vapordeposition process using a reactor as shown in FIGS. 1-3, with ring 52and with a wafer carrier having a sharp edge at its periphery. Thethickness of the deposited layer is stated as normalized thickness,i.e., a ratio of the thickness at each radial position to the thicknessat a radial position 190 mm from the center line. In each case, thewafer carrier has a diameter of 465 mm, so that the peripheral surfaceof the wafer carrier is disposed at a radial distance of 232.5 mm fromthe central axis. The vertical line 104 at approximately 223 mm radialdistance from the center line represents the radial location of theouter-most points of wafers on the carrier if the carrier is configuredto accommodate 54 wafers each having a diameter of two inches. Verticalline 106 at approximately 127 mm radial distance represents the radiallocation of the outer-most points on the wafers if the carrier isarranged to accommodate 6 six-inch diameter wafers. Curve 100 for theconventional reactor of FIG. 4 shows that, at line 104, the normalizedthickness is in excess of 1.1. By comparison, curve 102 indicates anormalized thickness of approximately 1.02 at line 104. Stated anotherway, if the wafer carrier is arranged to accommodate 54 two-inchdiameter wafers, the system without the ring will yield wafers havingsome points with thickness approximately 12% greater than other pointson the same wafers, whereas the system with the ring will yield waferswith deposited layers of substantially thickness uniform to within about2%. Further, if the wafer carrier is configured to hold 6 six-inchdiameter wafers, the system without the ring will yield wafers havingdeposited layer thickness variations in excess of 40%, i.e., anormalized thickness of 1.4 at vertical line 106. By contrast, curve 102indicates a thickness variation of approximately 7% at line 106, stillwithin acceptable limits for many applications. Therefore, the systemwith the ring can be more readily used to process 6 six-inch wafers,using a wafer carrier of the same diameter as the system without thering.

Thus, the improvement afforded by the ring allows construction of awafer carrier having pockets close to the periphery of the carrier,while still providing uniform treatment of the wafers. The wafer carrier340 depicted in FIG. 7 has a circular body with a generally planar topsurface 364 and a peripheral surface 366. The body has a fitting 367adapted to mate with the spindle of a processing apparatus, such as thespindle 34 of the processing apparatus shown in FIG. 1. The fitting canbe of any configuration; for a spindle having a conical fitting 38 asdepicted in FIG. 1, the fitting of the carrier typically is a conicalopening in the bottom of the body. The wafer carrier has wafer-holdingelements in the form of pockets 372, each adapted to hold a wafer. Eachof the pockets 372 lies close to the peripheral surface 366. Thus, thedistance X between the outermost part of each pocket and the peripheralsurface 366 is less than about 5 mm. Placement of the pockets so closeto the peripheral surface has not been acceptable heretofore due touneven thickness. The distance between the pocket and the peripheralsurface is measured between the pocket and the edge where the topsurface joins the peripheral surface. The body of carrier 340 may haveany diameter, but desirably has a diameter greater than 300 mm. In oneexample, the carrier has a diameter of about 465 mm and has six pocketsas depicted in FIG. 7, each pocket being adapted to hold one six-inchdiameter wafer. The carrier may include a greater number ofsmaller-diameter pockets, and the pockets may be arranged with only someof the pockets disposed at the outside of the body, near the peripheralsurface.

Ring 52 also acts as a thermal barrier between the periphery of thewafer carrier and the adjacent wall surface of the chamber defined byshutter 18. Typically, the wafer carrier is maintained at a temperaturesubstantially higher than the walls of the reactor. For example, thewafer carrier may be maintained at a temperature on the order of1000-1200° C. or higher, whereas the walls of the reactor may bemaintained at temperatures below 100° C. There is significant radiantheat transfer between the edge of the wafer carrier and the adjacentwall surface. This tends to make the edge region of the wafer carriercooler than other regions of the wafer carrier, and thus makes thewafers in the edge region cooler as well. Such non-uniform temperaturedistribution can result in non-uniform rates of reaction and non-uniformcomposition of a deposited layer. Although this effect can becounteracted to some degree by configuring the heater 42 to apply moreheat to the region of the wafer carrier near the periphery, it isdesirable to reduce this effect. Ring 52 acts as a radiation barrier andblocks direct radiation from the peripheral surface of the wafer carrierto the wall surface of the chamber defined by shutter 18. This helps tomaintain a uniform temperature distribution over the wafer carrier,which in turn, promotes uniformity of process conditions over allportions of the wafers.

To further enhance the insulating effect of the ring, ring 52 may beprovided with additional features which help to minimize heat conductionbetween the interior and exterior surfaces of the ring. For example, asseen in FIG. 6, ring 152 has a cross-sectional shape generally in theform of an inverted U, with a hollow interior space 153. The hollowinterior space reduces heat conduction between interior surface 158 andperipheral, or exterior, surface 156. Preferably, the top or upstreamsurface 154 of the ring remains as a continuous, unbroken surface. Space153 may be open on its downstream end, as depicted in FIG. 6, or may beclosed on its downstream end. By reducing heat conduction betweensurfaces 158 and 156, the ring depicted in FIG. 6 further impedes heattransfer between the wafer carrier 40 and shutter 18.

In a further variant, ring 52 can be formed from a plurality ofconcentric rings, each of which may be formed from the same material orfrom different materials. For example, the concentric rings can beformed from refractory materials, such as graphite, silicon carbide,and/or silicon coated graphite, as well as refractory metals, such asmolybdenum, rhenium, tungsten, niobium, tantalum, and alloys thereof.The size and number of concentric rings, as well as the materials thatmake up the ring 52, can be varied or adjusted depending upon the typeof reactor and/or reactions that take place within the reactor. In yetanother variant, the ring 52 may incorporate a heater, such as anelectrical resistance heater. For example, where the ring is formed as acomposite of multiple rings, one or more of the multiple rings mayconstitute a heating element. In one variant, the ring closest to theperipheral surface 66 of the wafer carrier 40 may be the heatingelement. Heating of the ring can be used to control the temperature ofthe wafer carrier edge. The heater incorporated in the ring can becontrolled by a feedback control system (not shown), which is sensitiveto the temperature of the wafer carrier 40 near the edge as, forexample, a control system using one or more pyrometers to monitor thewafer carrier temperature. The dimensions of rings according to theseand other variants can be selected as discussed above in connection withring 52, (FIGS. 1 and 2). Thus, here again the ring desirably is as wideas practicable and desirably provides the minimum practicable clearancearound the wafer carrier so as to minimize the size of the gap betweenthe wafer carrier and the ring.

The apparatus depicted in FIG. 8 is similar to that depicted in FIG. 1,except that the ring 252 has an upstream or top surface 254 in the formof a portion of a cone concentric with the central axis. Surface 254slopes upwardly in the radially outward direction, away from the centralaxis of the chamber. Stated another way, the juncture between theupstream or top surface 254 and the outer peripheral surface 256 liesupstream of the juncture between the upstream surface 254 and theinterior surface 258 of the ring. The vertical rise V of the top surfacebetween interior surface 258 and exterior surface 256 desirably isbetween about 1-2 mm. This upward slope helps to further suppressconvergence of streamlines in the region over the ring and over theperipheral region of wafer carrier 40. The shutter 218 may be modifiedslightly from that shown in the other figures so as provide the sameclearance C between the ring and the nearest portion of the shutter orwall structure. In a further variant, an upwardly sloping top surfacemay be provided as a surface of revolution of a curved generatrix aboutthe central axis. Thus, in this embodiment, the upstream or top surfacewould not define a straight line, but instead would define an upwardlysloping curved line.

Top surface 254 may alternatively slope downward in the radially outwarddirection, i.e., away from the central axis of the chamber. In yetanother variation, upstream or top surface 254 may be hemispherical inshape or may be planar with ridges or ripples of various shapes acrossthe surface.

Other embodiments may include a wafer carrier 40, which itself may havea lip projecting upwardly around the periphery of the top surface of thewafer carrier. Wafer carriers having upwardly-sloping lips are describedin U.S. Patent Application Publication No. 2011/0215071, the disclosureof which is hereby incorporated by reference herein. In theseembodiments, the upwardly-sloping lip of the wafer carrier may becombined with an upwardly-sloping surface 254 of the ring 252, ashereinbefore disclosed. Such surfaces may be arranged so that, when thewafer carrier is mounted at the carrier location, the upwardly-slopingsurface of the carrier lip and the ring 252 are nearly continuous withone another and cooperatively define a composite upwardly-slopingsurface. These upwardly-sloping surfaces may provide further control ofdeposition rates near the edge of the wafer carrier.

In a variant of the arrangements discussed above, structures such asrollers and guide pins (not shown) can be mounted to ring 52 (FIGS. 1and 2) or to baffle 44 to provide further assurance against contactbetween wafer carrier 40 and ring 52. Baffle 44 can also be furthermodified to include bumps or ridges on the wall proximate to the rollersto assist in the alignment of ring 52 with the top or upstream surface64 of the carrier 40. In some instances, it may be desirable to have thetop or upstream surface of ring 52 higher or lower than the top orupstream surface 64 of the carrier 40. In other instances, it may bedesirable to have interior surface 58 of ring 52 higher or lower thanthe top or upstream surface of ring 52. This can be accomplished, forexample, by having the rollers located within baffle 44 resting on theaforementioned bumps or ridges.

In the embodiments discussed above, the ring is mounted to the shutter.However, this is not essential. For example, the ring can be mounted ona separate actuator, and can be moved independently of the shutter. Instill other embodiments, the wall structure of the reactor may notinclude a shutter. In this case, the ring is disposed between thelocation of the wafer carrier and the fixed wall structure of thereactor. In the embodiments discussed above, the ring is movablerelative to the fixed wall structure of the reactor, either with theshutter or independently, so that the ring can be moved out of the wayduring loading and unloading of wafer carriers from the chamber.However, this is not essential. If the configuration of the wafercarriers and the configuration of the elements used to move the wafercarriers into and out of the reactor permit, the wafer carriers can beinstalled and removed without moving the ring.

During CVD processes, often times film growth will occur on the reactionchamber parts in addition to the intended substrate surface. If notcleaned, the additional film growth on the reactor chamber parts willaffect the efficiency of the CVD process, resulting in lower thanexpected yields as well as additional maintenance on the reactorchamber. One way to remove the additional film growth on ring 52 is toheat ring 52 to a temperature which will flash heat off the additionalfilm growth. A heater configured for such purposes can be incorporatedinto the ring 52, as described above.

Yet another method may be to cause vibration of ring 52. This can beaccomplished by raising and lowering ring 52 simultaneously causing therollers in baffle 44 to roll over the bumps or ridges located thereon.Vibration can also be accomplished by attaching an ultrasonic transducerto the ring 52 (or a supporting element thereof).

Yet another method to clean the additional film growth from the ring orthe shutter (for example, shutter 18) is to provide for one or moreorifices in the shutter wall facing the interior of the reactor chamberand/or the top surface of the ring, thus allowing non-reactive gas toflow through the orifice(s) and blast the additional film growth off therespective surfaces.

As shown in FIG. 9 (where, with the exception of reference numerals 300,305, 310, 315, 320, and 325, the reference numerals are as describedabove), gas inlet 300 may feed gas through gas tube 320 which connectsto and supplies the gas to orifice 325. In this variant, orifice 325 mayalso exit through the wall of the shutter 18. Moreover, as noted, one ormore orifices 325 can be placed along various positions of the wall ofthe shutter 18, such orifice(s) 325 facing the interior of the reactionchamber.

A separate gas inlet 305 may also feed gas through gas tube 310, whichconnects to and supplies the gas to orifice 315. However, in thisvariant, unlike orifice 325, orifice 315 may exit through the top orupstream surface 154 of ring 152. As with above, one or more orifices315 can be placed within the top or upstream surface 154. Additionally,the orifice(s) 315 can take the shape of a continuous or semi-continuousslit around the circumference of the ring 152.

Orifices 315 and 325 can be used to clean the additional film growth onthe ring (or other surfaces) in sequential or simultaneous activation.Gases suitable for use include, for example, H₂, N₂, Ar, and other inertgases. The gas(es) can be introduced into the reactor at a temperatureranging from about room temperature up to about 1600° C.

An alternate variation to orifices 315, 325 as set forth above, would beto have a gas tube 322 (shown in dotted lines in FIG. 9) extend throughthe base plate of the reactor. Gas tube 322, in this embodiment, may bea flexible bellows tube and may serve to clean the additional filmgrowth from the ring 152 and/or the shutter 18 in a similar manner toorifices 315, 325.

Another use for the one or more orifices 315 in ring 152, besidescleaning, may be for passing purge gas through the orifice(s) 315 onring 152 during the growth process. In so doing, one may be able toadjust the height of the boundary layer in the localized region of thepurge gas by adjusting the flow rate of the gas, which may be used tocompensate for any height variations caused by installing the ring 152.That is, if a “taller” flow extender (ring 152) is required, i.e., onethat projects above the top planar surface 154 of the wafer carrier, ahigher gas flow rate may be used to push purge gas through the one ormore orifices 315 on ring 152. Conversely, if a boundary layer closer tothe “z” plane of the wafer carrier is desired, the gas flow rate may bedecreased. By being able to adjust the efficiency of ring 152 by raisingor lowering the gas flow rate through orifice(s) 315 to adjust theboundary layer, tool-to-tool matching is made simpler by eliminatingprecise height adjustments during installation of ring 152 into MOCVDsystems.

For variants of ring 152 in which the ring is formed from a plurality ofconcentric rings, the rings may expand at different rates duringoperation, depending on whether or not the rings are formed of differentmaterials. This effect may cause the additional film growth discussedabove to become dislodged from the upstream or top surface of thering(s).

In the embodiments discussed above, the ring, while in the operativeposition, also remains stationary during processing of the wafers. Inother embodiments, the ring can be rotated around the central axisduring processing. For example, the ring may be mounted to a spindle sothat the ring can be rotated around the central axis during processingby a separate rotary drive. A diagrammatic view of this embodiment isshown in FIG. 10. The ring may rotate in the same direction as the wafercarrier and spindle, or in the opposite direction.

As shown in FIG. 10, assembly 200 has an outer spindle 134 and an innerspindle 168, which are connected to a rotary drive mechanism 136. Innerspindle 168 may have a fitting 138 at its upstream end. The fitting maybe arranged to releasably engage and hold a wafer carrier 440 at acarrier location similar to that depicted in FIG. 1. Wafer carrier 440may include numerous pockets 172, each of which is arranged to hold awafer 174 so that the top surface of the wafer is substantially coplanarwith top surface 464 of wafer carrier 440. Outer spindle 134 may alsohave a fitting 238 at its upstream end. The fitting may be arranged toengage (in other embodiments, releasably engage) support 360 to whichring 352 is mounted. The rotary drive 136 is designed to allow forindependent rotation of inner spindle 168 and outer spindle 134,permitting the wafer carrier to rotate in the same direction as thering, in opposite directions, or to allow the ring to remain stationaryas the wafer carrier rotates.

Support 360 may take many shapes. In some instances, support 360 may bea susceptor or may be a series of support arms which extend radiallyfrom outer spindle 134 to just beyond the outer edge of wafer carrier464, at which point ring 352 is mounted on the support arms. Support 360may be made from any suitable material capable of withstanding the hightemperatures inside the reaction chamber of an MOCVD reactor and, at thesame time, permit appropriate heat transfer from heater 142 to wafercarrier 440. Heater 142 of assembly 200 may be mounted and configured ina manner like heater 42 described hereinabove.

FIG. 11 shows a top plan view of assembly 200 of FIG. 10, with support360 shown in phantom lines. Gap 170 between the peripheral edge 466 ofwafer carrier 440 and interior surface 358 of ring 352 is similar insize to that of gap 70 described previously.

Additionally, rotary drive 136 may, apart from permitting rotation ofring 352, allow for separate height adjustments of top surface 464 ofwafer carrier 440 in relation to top surface 564 of ring 352. For somegrowth process steps, it may be beneficial to have top surface 464 ofwafer carrier 440 essentially coplanar with top surface 564 of ring 352.In other growth process steps, it may be beneficial to have top surface464 of wafer carrier 440 higher or lower than top surface 564 of ring352. Ring 352 can also take the shape and characteristics of ring 252 asdescribed above.

In yet another embodiment, the ring may be disposed within the chamberand arranged to releasably engage the outer edge of the wafer carrier sothat the ring, in effect, becomes a temporary part of the wafer carrierduring operation.

The materials of construction of the reactor elements, and thecomposition of the treatment gasses, may be conventional. For example,the wafer carrier may be formed in whole or in part from refractorymaterials such as graphite, silicon carbide, and silicon carbide coatedgraphite, whereas elements such as the ring may be formed of similarmaterials or from refractory metals such as molybdenum. Metals used forthe ring optionally may be blackened to increase the emissivity of themetal. The treatment gasses may be, for example, gasses selected toreact in a chemical vapor deposition reaction or gasses selected to etchor otherwise treat the surfaces of the wafers.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A reactor comprising: (a) a chamber having a wall structure definingan interior surface; (b) a spindle disposed within the chamber androtatable about an upstream-to-downstream axis, the spindle beingadapted to support a wafer carrier for rotation about the axis so that atop surface of the carrier faces in the upstream direction at a carrierlocation; (c) a ring mounted within the chamber, the ring having a topsurface facing in the upstream direction, the ring being constructed andarranged so that when the reactor is in an operative condition, the ringclosely surrounds the wafer carrier supported on the spindle and the topsurface of the ring is substantially continuous with the top surface ofthe carrier.
 2. A reactor as claimed in claim 1 further comprising: agas inlet element communicating with the chamber upstream of the carrierlocation and a gas exhaust communicating with the chamber downstream ofthe carrier location, the ring having a peripheral surface facingoutwardly away from the axis, the ring being arranged so that when thereactor is in an operative condition, there is a gap between theperipheral surface of the ring and the interior surface of the chamber.3. A reactor as claimed in claim 1 wherein the spindle is adapted toreleasably engage the wafer carrier and the chamber has an opening forinsertion and removal of wafer carriers.
 4. A reactor as claimed inclaim 3 wherein the chamber wall structure includes a fixed wallstructure and the ring is movable relative to the fixed wall structurein the upstream and downstream directions.
 5. A reactor as claimed inclaim 4 wherein the chamber wall structure includes a shutter defining aportion of the interior surface, the shutter being movable in theupstream and downstream directions relative to the fixed wall structureof the chamber, and wherein a gap is defined between the peripheralsurface of the ring and the shutter.
 6. A reactor as claimed in claim 5wherein the ring is attached to the shutter for movement therewith inthe upstream and downstream directions.
 7. A reactor as claimed in claim1 wherein the ring is fixed against rotation around the axis.
 8. Areactor as claimed in claim 1 wherein the ring is rotatable about theaxis.
 9. A reactor as claimed in claim 8 wherein the ring is rotatableabout the axis independently of the spindle.
 10. A reactor as claimed inclaim 1 further comprising a heater within the chamber, the heater beingadapted to heat the wafer carrier supported on the spindle.
 11. Areactor as claimed in claim 1 wherein the ring defines a hollow spacebelow the top surface of the ring.
 12. A reactor as claimed in claim 1wherein the ring is formed from a plurality of concentric rings.
 13. Areactor as claimed in claim 12 wherein each of the concentric rings iscomprised of the same or different material.
 14. A reactor as claimed inclaim 1 wherein the ring acts as an insulater for heat generated by aheater included in the chamber.
 15. A reactor as claimed in claim 12wherein at least one of the plurality of concentric rings includes aheating element.
 16. A reactor as claimed in claim 1 wherein the topsurface of the ring comprises at least one orifice for introducing a gasinto the chamber.
 17. A reactor as claimed in claim 5 wherein theshutter comprises at least one orifice for introducing a gas into thechamber.
 18. A method of processing wafers comprising the steps of: (a)positioning a wafer carrier inside a reaction chamber so that a ringwithin the chamber surrounds the carrier, so that top surfaces of thecarrier and ring facing in an upstream direction are substantiallycontinuous with one another and so that surfaces of wafers disposed onthe carrier face in the upstream direction; and (b) directing one ormore treatment gasses in a downstream direction opposite to the upstreamdirection onto the top surfaces of the wafer carrier and wafers whilerotating the wafer carrier and wafers around an upstream-to-downstreamaxis, so that the treatment gasses flow outwardly over the top surfaceof the carrier and the surfaces of the wafer, and flow outwardly fromthe top surface of the carrier over the top surface of the ring.
 19. Amethod as claimed in claim 18 further comprising exhausting gasses fromthe chamber downstream from the ring so that the gasses flowingoutwardly over the top surface of the ring pass downstream within a gapbetween the ring and a wall of the chamber.
 20. A method as claimed inclaim 19 further comprising removing the wafer carrier from the chamberafter the directing step, and repeating the aforesaid steps with anotherwafer carrier having new wafers disposed thereon.
 21. A method asclaimed in claim 20 further comprising moving the ring upstream ordownstream after the directing step and before the step of removing thecarrier.
 22. A method as claimed in claim 21 wherein the step of movingthe ring after the directing step includes moving a shutter mechanicallyconnected to the ring from an operative position to an open position inwhich the shutter does not occlude an opening in the chamber wall, andthe step of removing the wafer carrier includes removing the carrierfrom the chamber through the opening while the carrier is in the openposition.
 23. A method as claimed in claim 22 wherein the shutter andthe ring cooperatively define the gap when the shutter and ring are inthe operative position.
 24. A method as claimed in claim 18 furthercomprising the step of heating the wafer carrier, wherein the ring actsto impede heat transfer from the wafer carrier to the wall of thechamber.
 25. A wafer carrier comprising a body having a circular topsurface, a peripheral surface bounding the top surface and a fittingadapted to engage a spindle of a wafer processing reactor so that thetop surface and peripheral surface are concentric with the spindle, thebody further defining a plurality of pockets each adapted to hold awafer, the pockets including outer pockets adapted to hold wafers sothat portions of the wafers lie within about 5 mm of the peripheralsurface.
 26. A wafer carrier as claimed in claim 25 wherein the topsurface has a diameter of about 465 mm and the pockets include at leastsix pockets, each adapted to hold a wafer having a diameter of about 6inches.